Recently, semiconductor memory devices such as NAND flash memories have been included in various electronic devices. Due to requirements for providing such electronic devices with multiple functions, a semiconductor memory device has been required to have large memory capacity. Accordingly, memory cells of the semiconductor memory device have been required to be miniaturized.
A NAND flash memory will be described as an example. In a typical NAND flash memory, a plurality of memory cell transistors are electrically connected in series to form a NAND cell unit. One end of the NAND cell unit is electrically connected to a bit line via a select gate transistor, and the other end of the unit is electrically connected to a source line via another select gate transistor.
Recently, due to miniaturization of a memory cell array, densification and integration in a memory have been advanced. Accordingly, bit line contacts arranged between drain-side select gates have also been required to be reduced in size. However, it is difficult to simply reduce the size of the contacts and the space between the contacts, because they may cause the short of adjacent contacts, decrease in breakdown voltage between the contacts, increase in contact resistance due to the miniaturization, optical interference in lithography due to the shortened distance between the contacts, and the like.
Therefore, attempts to solve these problems have been recently made by using a staggered contact arrangement. An example of the staggered contact arrangement is a two-contact staggered arrangement in which two contacts are the repetition unit of a staggered structure. However, due to the advance of the miniaturization of the memory, the distance between adjacent contacts tends to be shorter, so that even the two-contact staggered arrangement cannot solve the above problems. Therefore, it can be considered to use a multiple-contact staggered arrangement in which three or more contacts are the repetition unit of the staggered structure. However, if the multiple-contact staggered arrangement is adopted, the distance between the select gates is increased due to this arrangement, resulting in the increase of the chip area.
Therefore, it is required to provide such a contact arrangement that can suppress the increase of the chip area while the short of the contacts and the decrease in breakdown voltage between the contacts are suppressed.